1. Field of the Invention
The present invention relates to a surface-mountable antenna which is mounted on a substrate at a surface of its dielectric substrate, to be supplied with electricity from a feeding part provided on the substrate, and more particularly, it relates to an improved surface-mountable antenna with controllable directivity.
2. Description of the Background Art
With the recent prevalence of car telephones and portable telephones, there is a great need for miniaturization of antennas for transmitting/receiving high-frequency signals for such telephones.
FIGS. 5A and 5B are perspective views showing an antenna 10 for a communication device such as a portable telephone and the body 80 of the communication device respectively. The antenna 10 is a dielectric-loaded monopole antenna. In this antenna 10, a through hole 30 is formed in a cylindrical dielectric body 20, and a radiating electrode 40 which is made of Cu, for example, is formed on the inner periphery of the through hole 30. Further, a male connector 60 is mounted on one end surface of the dielectric body 20. This male connector 60 is connected with a female connector 70 which is provided on a body 80 of the communication device, thereby enabling the supply of electricity to the radiating electrode 40 and transmitting/receiving of high-frequency signals.
In such a communication device, however, the antenna 10 is provided outside the body 80 of the communication device, which hinders the miniaturization of the communication device, and further, an external force can act directly on the antenna 10. Thus, there is a probability of causing problems such as a reduction in mechanical strength and durability, and changes of its electrical characteristics.
In such a communication device, further, the high-frequency signals are transmitted/received through the connectors 60 and 70, leading to problems such as an increase in insertion loss and a change of the resonance frequency.
In addition, the number in components of such a communication device is increased due to employment of the connectors 60 and 70, to disadvantageously reduce its workability and increase its cost.
To this end, there has been developed a surface-mountable antenna 11 which is directly mounted on a substrate with no employment of connectors, as shown in FIG. 6.
In this surface-mountable antenna 11, a through hole 33 is formed in a prismatic dielectric substrate 22 between first and second end surfaces thereof, and a radiating electrode 44 is formed on the inner peripheral surface of this through hole 33. Further, an end electrode 99 is formed on the first end surface of the dielectric substrate 22. This end electrode 99 is connected with the radiating electrode 44.
A substrate 100 is enclosed in a case for the body of a communication device or the like, thereby mounting the surface-mountable antenna 11 in the case. This substrate 100 is provided on its mounting main surface with a feeder line 140 serving as a feeding part for the surface-mountable antenna 11, and signal processing circuits (not shown) such as a transmission circuit and a receiving circuit.
The surface-mountable antenna 11 is placed on the substrate 100 on its mounting side surface, to be connected and fixed to the substrate 100 by solder and an adhesive (not shown), for example, so that the end electrode 99 faces the feeder line 140.
Further, fixing electrodes 88 are formed along side and bottom surfaces of the dielectric substrate 22. The surface-mountable antenna 11 is connected and fixed to the substrate 100 by solder and an adhesive (not shown) similarly to the above, so that the fixing electrodes 88 face fixing conductors 180 which are formed on the mounting main surface of the substrate 100.
As compared with the conventional dielectric-loaded antenna, this surface-mountable antenna 11 is advantageous in that the same can be directly surface-mounted on the substrate 100 with no requirement for connectors.
However, the conventional monopole type surface-mountable antenna has the following problem, since its directivity cannot be controlled: When the antenna is applied to a portable telephone, for example, this antenna is integrated into the device as a matter of course. In the conventional antenna, therefore, it is impossible to avoid problems such as mutual interference between systems being used and generation of radio waves toward another device or the human body.
In the conventional monopole type surface-mountable antenna, further, it is difficult to attain a high gain due to dispersion of the directivity.